Ink-jet printing is a non-impact priming process in which droplets of ink are deposited on print media, such as paper, transparency film, or textiles, in response to an electrical signal. In thermal ink-jet printing, a resistor element in a chamber is supplied with ink via a plenum, which is connected to an ink reservoir. A plurality of such resistor elements are arranged in a particular pattern in the printhead of an ink-jet print cartridge. Each resistor element is associated with a nozzle in a nozzle plate. In operation, each resistor element is connected to a microprocessor, whose signals direct one or more resistor elements to heat up rapidly. The heat creates a vapor bubble in the chamber, which expels ink through the associated nozzle onto the print medium. The firing of a plurality of such resistor elements in a particular order forms alphanumeric characters, area fills, and other patterns on the print medium. In piezoelectric ink-jet printing, ink droplets are ejected due to the vibrations of piezoelectric crystals, again, in response to electrical signals generated by the microprocessor.
Ink-jet printers offer low cost, high quality printing with relatively noise-free operation. As such, ink-jet printers have become a popular alternative to other types of printers used with computers, such as the more expensive laser printers. However, ink-jet printers are presently incapable of matching the level of throughput generated by laser printers due in large part to the relatively slow dry time of ink-jet inks as well as to printer slow-down deriving from the resident bleed control algorithms in the printer. With particular regard to bleed control, there is a tendency among ink-jet inks to bleed into one another when printed in various colors on paper substrates. Bleed occurs as colors mix both on the surface of the paper substrate as well as within the substrate itself. In response to this problem, ink-jet primers commonly employ bleed control algorithms in an attempt to provide a border between colors that is clean and free from the invasion of one color into another; however, this slows down the primer. In order to increase the level of throughput generated by ink-jet primers, the dry time of ink-jet inks should be improved, preferably in a manner that also effects bleed control.
Various solutions to improve the dry time of ink-jet inks have been proffered. Some solutions involve changing the ink environment to hasten dry time. For example, heated platens and other heat sources have been employed to achieve faster dry times. However, heated platens add cost to the primer, which is generally not cost effective.
Other solutions involve adding various components to the ink-jet ink composition to achieve fast dry times. For example, surfactants have been added to dyebased ink-jet ink compositions to improve dry time by increasing the penetration rate of the ink into the paper; see, e.g., U.S. Pat. No. 5,106,416 entitled "Bleed Alleviation Using Zwitterionic Surfactants and Cationic Dyes", issued to John Moffatt et al; U.S. Pat. No. 5,116,409 entitled "Bleed Alleviation in Ink-Jet Inks", issued to John Moffatt; and U.S. Pat. No. 5,133,803 entitled "High Molecular Weight Colloids Which Control Bleed", issued to John Moffatt, all assigned to the same assignee as the present application. Notably, bleed control is also achieved with the addition of surfactants. Although increasing the penetration rate of the ink reduces its dry time, an undesirable reduction in edge acuity may also result from the addition of surfactants. Moreover, the addition of surfactants to ink-jet inks is known to negatively affect drop ejection characteristics. More particularly, the low surface tensions of surfactant-containing inks could cause puddles on the nozzle plates of the printhead, leading to poor drop ejection characteristics. Short-chain alcohols have also been added to ink-jet ink compositions to hasten dry times, but, depending upon the particular print medium, the addition of short-chain alcohols may adversely affect print quality due to increased edge roughness. Thus, the improved dry times achieved with the addition of surfactants and short-chain alcohols are often achieved at a sacrifice to print quality.
Accordingly, a need exists for ink compositions for use in ink-jet printing that dry rapidly upon impact with the print medium and evidence reduced bleed. However, the print quality of the ink compositions must not be sacrificed to achieve faster dry times.